1. Field of the Invention
The present invention relates to wireless communication, and more particularly, to a method and apparatus for transmitting a training signal in a Wireless Local Area Network (WLAN) system.
2. Related Art
With the recent development of information communication technology, a variety of wireless communication techniques are being developed. From among them, a WLAN is a technique which enables wireless access to the Internet at homes or companies or in specific service providing areas through mobile terminals, such as a Personal Digital Assistant (PDA), a laptop computer, and a Portable Multimedia Player (PMP), on the basis of radio frequency technology.
Since Institute of Electrical and Electronics Engineers (IEEE) 802 (i.e., the standard organization of WLAN technology) was set up on February, 1980, lots of standardization tasks are being performed.
The initial WLAN technology was able to support the bit rate of 1 to 2 Mbps through frequency hopping, band spreading, and infrared communication by using a 2.4 GHz frequency band in accordance with IEEE 802.11, but the recent WLAN technology can support a maximum bit rate of 54 Mbps by using Orthogonal Frequency Division Multiplex (OFDM). In addition, in the IEEE 802.11, the standardization of various techniques, such as the improvements of Quality of Service (QoS), the compatibility of Access Point (AP) protocols, security enhancement, radio resource measurement, wireless access vehicular environment for vehicle environments, fast roaming, a mesh network, interworking with an external network, and wireless network management, is put to practical use or being developed.
Furthermore, as a recent technology standard for overcoming limits to the communication speed considered as vulnerabilities in the WLAN, there is IEEE 802.11n which has recently been standardized. The object of the IEEE 802.11n is to increase the speed and reliability of a network and to expand the coverage of a wireless network. More particularly, the IEEE 802.11n is based on a Multiple Inputs and Multiple Outputs (MIMO) technique using multiple antennas on both sides of a transmitter and a receiver in order to support a High Throughput (HT) having a data processing speed of 540 Mbps or higher, minimize transmission errors, and optimize the data rate. Furthermore, the IEEE 802.11n may use not only a coding method of transmitting several redundant copies in order to increase data reliability, but also an Orthogonal Frequency Division Multiplex (OFDM) method in order to increase the data rate.
With the spread of the WLAN being activated and applications using the WLAN becoming diverse, there is a need for a new WLAN system for supporting a higher throughput than the data processing speed supported by the IEEE 802.11n standard. A Very High Throughput (VHT) WLAN system is one of IEEE 802.11 WLAN systems recently proposed in order to support the data processing speed of 1 Gpbs, or higher. The term ‘VHT WLAN system’ is arbitrary, and a feasibility test for MIMO and a system using a channel bandwidth of 80 MHz or higher is in progress in order to provide the throughput of 1 Gpbs or higher.
In order to handle an increase of the number of spatial streams that can be used to send data and to support MIMO transmission for multiple users, a method of generating and sending a training signal for the channel estimation of spatial streams between a transmission station and a reception station needs to be taken into consideration.